The vast majority of conventional aircraft have a pair of horizontal stabilizers mounted at the rear of the fuselage. The horizontal stabilizers provide the aircraft with longitudinal pitch stability in flight. In larger aircraft, for example commercial passenger aircraft, the horizontal stabilizers are trimmable i.e. moveable, to allow adjustments to the longitudinal pitch of the aircraft to be made during flight. Such adjustments may be necessary to take into account the changing center of a gravity of the aircraft as the fuel load carried is reduced through fuel consumption.
One known mechanical actuator that is used to move the horizontal stabilizers for trimming purposes is a ballscrew and corresponding ballnut. A ballscrew is a cylindrical shaft having an integral screw thread formed on it. The shaft is rotated using a motor drivingly connected to one end of the shaft. Mounted on the ballscrew is a ballnut that has a corresponding thread formed on its interior. The horizontal stabilizer is mechanically connected to the ballnut. Rotation of the ballscrew causes the ballnut to translate linearly along the ballscrew and thus transmit the linear motion to the horizontal stabilizer via the coupling mechanism.
It is often a requirement of aircraft components and mechanisms that the mechanisms or components have redundancy in the event of a failure. By redundancy it is meant that in the event of failure of a primary component or mechanism there is at least a secondary component or mechanism provided to allow safe operation of the mechanism to continue or to hold it in a fixed position.